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BP: Fachverband Biologische Physik

BP 17: Microswimmers (joint DY/BP)

BP 17.3: Talk

Tuesday, April 1, 2014, 10:00–10:15, ZEU 146

Hydrodynamics of spherical microswimmers in a quasi-2D geometry — •Andreas Zöttl and Holger Stark — TU Berlin

Microorganisms like bacteria, sperm cells or algae live in aqueous environments and their motion is therefore governed by low-Reynolds-number hydrodynamics, but also influenced by thermal and biological noise. Experiments with artificial microswimmers, which are used to study collective motion of self-driven particles out of equilibrium, are often performed in a quasi-2D geometry or in thin films, where dynamic clustering and motility-induced phase separation is observed.

Motivated by recent experiments of active colloids and emulsion droplets in confinement, we also study the collective motion of spherical squirmers moving in a quasi-2D geometry by means of multi-particle collision dynamics. Hydrodynamic near-field interactions between swimmers lead to hydrodynamic rotational diffusion, while hydrodynamic interactions between the channel walls and swimmers strongly influence the preferred swimmer orientations and therefore the formation of hexagonal clusters. Neutral squirmers in particular separate into a gas-like and a crystalline phase which we characterize by a structural order parameter. Varying the density of the swimmers from low to high area fraction results in a steep increase of the order parameter at the critical density, accompanied by strong fluctuations indicating a non-equilibrium phase transition which is absent for strong pullers and pushers.